This invention relates to a method and a device for suspending particles in a fluid, especially for suspending living cells in a small volume of fluid.
Biological samples like cells need to be suspended in a liquid for culturing or analyzing purposes. Such suspensions are stable for a short time until the sample starts sedimentation which makes remixing necessary. Mixing suspensions of biological samples is a long known technique for which various devices are commercially available. Design and function of the mixing devices depend on the nature and volume of the sample to be mixed, the size and form of the mixing vessel and subsequent processing steps of the sample.
Besides mixing a suspension by a stirrer coupled to an electric motor, magnetic cell stirrers are in common use. To provide larger volumes of suspensions, EP53869, U.S. Pat. No. 3,854,704 and U.S. Pat. No. 3,572,651 disclose a magnetic cell stirrer wherein the suspension is mixed by a rotating magnet placed in the mixing vessel via a flexible shaft. The magnet inside of the vessel is forced into rotational movement by a magnet outside of the vessel which in turn is rotated by a conventional magnetic stirring apparatus. The flexible shaft is attached to the mixing vessel and can not be removed or inserted into the vessel.
U.S. Pat. No. 3,780,992 discloses a vibrating pipette, wherein a mechanical oscillator is attached to the pipette as a mixing rod.
U.S. Pat. No. 4,204,774 describes a magnetic cell stirrer wherein the suspension is mixed by a rotating stirring rod. The rod is rotated in a circular path in the mixing vessel by an eccentric means and a synchronous motor. The magnetic cell stirrer is firmly attached to the mixing vessel and can not be removed or inserted into the vessel.
Biological samples for testing or analyzing purposes are usually provided in small volumes ranging from less than 10 μl to 5 ml. For example, commonly used microplates provide up to 1024 wells having a volume less than one milliliter. Mixing of such small volumes by mere stirring is difficult due to capillary forces and a low surface to volume ratio of the vessel resulting in adhesion of the liquid to the vessel walls. Mixing by stirring generates a single stream of liquid moving in a circle adjacent to the vessel wall having low turbulence and accordingly a low mixing efficiency.
In this respect, US2008/0078257 discloses a mixing process, wherein a magnetic mixing rod is moved by a second magnet rotating around the mixing rod.
Suspensions in microplate wells are therefore usually mixed by shaking or vibrating the whole microplate i.e. all wells simultaneously. Shaking cell suspensions in a microplate might result in cell loss since cells can adhere to walls of a well above the surface of the liquid
It would be beneficial to provide small volume suspensions of biological samples without stirring the fluid in circles and/or the need to shake/vibrate other vessels. It would furthermore be desirous to provide a device to generate such suspensions which can be implemented in analytical devices like a FACS machines for sample preparation upstream of the cell analysis device.
Surprisingly, it was found that particle suspensions, especially in small volume containers can be efficiently homogenized by a device wherein a magnetic mixing rod is moved in an oscillating manner by interaction with a rotating second magnet.